The present invention relates to new block copolymers comprising a first sequence formed with monomeric units derived from conjugated dienes or vinyl substituted aromatic hydrocarbons other than alpha-methylstyrene, and a second sequence formed with monomeric units derived from acrylic esters. The present invention is also concerned with a process for making these new block copolymers by anionic polymerization.
The making of block copolymers by anionic polymerization as disclosed in W. H. James and D. C. Allport, Block Copolymers, 1973, Applied Science Publishers Ltd, pages 62 to 104 hereby incorporated by reference, is well known in the art. It is also known that not only do the block copolymers styrene-acrylic esters obtained in accordance with the usual processes have a branched structure, but in addition their copolymerization is strongly hindered. These drawbacks are due to a side reaction between the ester function and the carbanion which ends the first sequence. In order to avoid this side reaction, it has already been proposed to add diphenyl-1,1-ethylene at the end of the polymerization of the first sequence; however this latter additive is expensive and its purification difficult. There is therefore a need for block coploymers which avoid the above-mentioned side reaction and consequently the formation of a branched sturcture, but which can be obtained without the introduction of an additive such as diphenyl-1,1-ethylene.